The present invention relates to methods for sintering powder metallurgy parts and more particularly to methods for sintering steel powder metallurgy parts in nitrogen based atmospheres.
In producing powder metallurgy parts it is common to pass `green` compacts through a sintering furnace typically comprised of a preheat section, high temperature (hot zone) section and a cooling section which sections are supplied with a controlled atmosphere. The most widely used controlled atmosphere used with sintering furnaces has heretofore been endothermic gas which is typically comprised of 40-45% nitrogen, 19-21% carbon monoxide, 32-39% hydrogen and 0.2-0.6% methane with the foregoing gas mixture having a dewpoint between +20.degree. to +50.degree. F. Endothermic gas has been widely used for sintering due to its low cost and tolerant nature which enables powder metallurgy parts to be sintered under relatively high dewpoints. In addition, the carbon available from the minor methane fraction of endothermic gas counteracts a decarburizing effect caused by the reaction between oxygen carried into the sintering furnace by such parts and added graphite. One major disadvantage attributable to endothermic gas is that substantial quantities of natural gas or other hydrocarbon sources are required for its production. Typically, 450 SCF of natural gas are required to produce each 1,000 SCF of endothermic gas and consequently, large quantities of a relatively scarce and expensive resource, natural gas, are required for producing endothermic gas. In addition, sintering under endothermic gas at temperatures of 2050.degree. F or so tends to decarburize powder metallurgy parts due to the high dewpoint (moisture content) of endothermic gas. Furthermore, the chemistry of natural gas utilized to generate endothermic gas is not consistent and in order to effectively sinter powder metallurgy parts, relatively expensive analytical instruments and control devices must be utilized.
Other atmospheres such as purified exothermic gas, which is generated from burning approximately 6 parts of air with one part of natural gas and subsequently removing CO.sub.2 and moisture, has been utilized. This gas is essentially comprised of 75% nitrogen, 11% carbon monoxide and 13% hydrogen, with a dewpoint of approximately -40.degree. F. Although purified exothermic gas does not require as much natural gas for its production as does endothermic gas, exothermic gas has been found suitable as a controlled sintering atmosphere only when the desired surface carbon content of a part being sintered or treated is less than 0.3%. Accordingly, purified exothermic gas is not preferred as a controlled atmosphere for use during sintering of powder metallurgy parts in which higher surface carbon concentrations are required.
Dissociated ammonia has also been used as controlled sintering atmosphere in sintering processes and generally exhibits a low dewpoint (typically -60.degree. F). This atmosphere (25 N.sub.2, 75 H.sub.2) is protective, reducing in nature and decarburization is not a problem as long as such dewpoint is maintained and relatively low oxygen concentrations exist in the furnace hot zone. However, as PM parts carry oxygen into the sintering furnace and as such furnaces will inevitably admit some oxygen into the hot zone through leakage, etc., atmospheres such as dissociated ammonia which contain no hydrocarbon constituents to counteract decarburization. Furthermore, as ammonia is generally a by-product of processes in which hydrocarbons such as natural gas are required and consequently, the production of dissociated ammonia relies upon natural gas as a feed stock.
Nitrogen gas alone has been utilized as a sintering atmosphere and in the event the dewpoint of such atmosphere can be maintained below -60.degree. F, sintering powder metallurgy steel parts will be generally effective. However, as virtually all furnaces have some leaks through which atmospheric air may enter, the effectiveness of nitrogen alone as a sintering atmosphere decreases due to the decarburizing effect of oxygen and other airborne contaminants. Although sintering atmospheres comprised of only nitrogen and 0-10% hydrogen have been utilized, in the event that any significant moisture is present, the hydrogen component of this atmosphere will act as a decarburizing agent and thereby causes significant reductions in surface carbon and hardness.
Accordingly, a clear need exists in the powder metallurgy sintering art for processes wherein controlled atmospheres are utilized to effectively sinter such parts without consuming substantial quantities of natural gas or other hydrocarbons.